<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    ul {
      list-style: none;
      margin: 0;
      padding: 0;
      display: flex;
      width: 100%;
      height: 200px;
      border: 1px solid #000;
      /* justify-content: space-around;
      flex-direction: column; */
      justify-content: center;
      align-items: center;
    }

    li {
      width: 200px;
      height: 50px;
      background-color: aqua;
      /* flex: 0 0 200px; */
    }

    /* li:nth-child(2) {
      flex: 1;
    } */
  </style>
</head>

<body>
  <ul>
    <li>1</li>
    <li>2</li>
    <!-- <li>3</li> -->
  </ul>
</body>

</html>